1. Field of the Invention
The invention relates to an electricity storage control apparatus that controls an electricity storage device mounted on a vehicle, and to a method of controlling electricity storage.
2. Description of the Related Art
An electricity storage device (battery) is used as a drive power source of a vehicle. For example, in a hybrid vehicle, the electricity storage device discharges stored electricity to drive a motor, and thus causes wheels to rotate when the driver starts driving or during full throttle acceleration.
It is known that the internal resistance of the battery increases as the battery is repeatedly charged and discharged, and therefore the battery power output gradually decreases.
In consideration of the foregoing, in the related art, a target state of charge of the battery is fixed at a relatively high value (60%) so that a battery power output larger than a reference battery power output corresponding to a maximum vehicle power output is obtained, as shown in FIG. 5A illustrating a relation between the vehicle use time and the battery power output, and in this way, the battery is designed so that the battery power output does not fall below the reference battery power output before the vehicle lifetime expires.
On the other hand, as a method of extending the lifetime of the electricity storage device, a focus is placed on the principle that the wider the variation range of state of charge is, the shorter the lifetime of the electricity storage device is. Based on this, in the related art, it has been proposed that a maximum state of charge be decreased and a minimum state of charge be increased (see paragraph 0046, for example, of Japanese Patent Application Publication No. 2003-297435 (JP-A-2003-297435)).
In Japanese Patent Application Publication No. 2003-47108 (JP-A-2003-47108), a method has been proposed that, when the occurrence of a memory effect is detected based on the voltage, current, temperature, etc. of the battery, the target state of charge be changed from 50%, which is a setting used under normal conditions, to a value near the maximum state of charge within an appropriate state-of-charge range (see paragraph 0035, for example). In JP-A-2003-47108, it is said that, in this way, the memory effect can be eliminated without overcharging or overdischarging the battery, which makes it possible to prevent degradation of the battery.
However, as shown in FIG. 6 illustrating a relation between the rate at which the battery degrades and the target state of charge, there is a problem that, when the target state of charge is increased, the rate at which the battery degrades gets faster because the output voltage is increased. Accordingly, as shown in FIG. 5B, if the target state of charge is fixed at 60%, the battery lifetime becomes shorter, and it contradicts the demand for a smaller-sized battery.
In a method described in Japanese Patent Application Publication No. 2003-297435 (JP-A-2003-297435), only the upper limit or lower limit of the variable range in which the target state of charge is changed is changed, and the target state of charge itself remains the same. Accordingly, the aforementioned problem cannot be solved.
A method described in Japanese Patent Application Publication No. 2003-47108 (JP-A-2003-47108) merely increases the target state of charge to reset the memory effect when the occurrence of a memory effect is detected. Therefore, the problem of the accelerated degradation rate of the battery, which arises when the target state of charge is fixed at a high value, cannot be solved.